DNA methylation is known to be involved in the repression of gene activity, but its actual role during development has not yet been elucidated. Using a very sensitive PCR assay it has been shown that the methylation pattern of the mouse genome undergoes dynamic changes in specific genes during embryogenesis and gametogenesis. The significance of these events and the mechanisms and rule by which methylation is altered will be studied both in vivo by injecting DNA into mouse zygotes and by transfection into embryonic tissue culture cells. Several genes in the mouse have been shown to undergo parental imprinting and it has been suggested that this selective expression of only one parental allele is mediated by the methylation pattern derived from the parents. This hypothesis will be tested by PCR analysis of methylation in the embryo and more directly through the use of transgenic mice. In a similar manner the role of methylation in the germ line inactivation and reactivation of the X chromosome will also be investigated. Following gastrulation, the DNA methylation pattern becomes fixed and is then faithfully passed on to all somatic cells. Tissue specific demethylation then occurs during differentiation. Using DNA gene transfer, it will be possible to evaluate the role of this demodification reaction and the cis and trans factors which provide the specificity.